Corrosion phenomena concerning reinforced concrete structures is known in the art. The steel reinforcement inserted in the concrete structures to improve their mechanical properties normally works in a passivation regime induced by the alkaline environment of the concrete. Nevertheless, after a certain time the ion migration across the concrete porous structure provokes a localised attack to the protective passivation film. Particularly worrying is the attack by chlorides, which are virtually present in all kinds of environments where the reinforced concrete structures are employed, and to an even higher extent where an exposure to brackish water (bridges, pillars, buildings located in marine zones), antifreeze salts (bridges and road structures in cold regions) or even seawater, for example, in the case of piers and docks, takes place. The critical value of chloride exposure has been estimated around 0.6 kg per cubic meter of concrete, beyond which the passivation state of the reinforcing steel is not guaranteed.
Another form of concrete decay is represented by the phenomenon of carbonatation, that is the formation of calcium carbonate by reaction of the lime of the cementitious mixture with atmospheric carbon dioxide. Calcium carbonate lowers the concrete alkali content (from pH 13.5 to pH 9) bringing iron to an unprotected state. The presence of chlorides and the simultaneous carbonatation represent the worst of conditions for the preservation of the reinforcing bar of the structures. The corrosion products of steel are more voluminous than steel itself, and the mechanical stress arising from their formation may lead to concrete delamination and fracturing phenomena, which translate into huge damages from the point of view of economics besides that of safety.
It is known in the art that the most effective method for indefinitely prolonging the lifetime of reinforced concrete structures exposed to the atmospheric agents, even in the case of remarkable salt concentrations, consists of cathodically polarising the steel reinforcement. In this way, the latter becomes the site of an oxygen cathodic reduction, suppressing the anodic corrosion and dissolution reactions. Such a system, known as cathodic protection of reinforced concrete, is practised by coupling anodic structures of various kinds to the concrete, with the reinforcement to be protected acting as a cathodic counterelectrode. The electrical currents involved supported by an external rectifier travel across the electrolyte, a porous concrete partially soaked with a salty solution. The installation of a cathodic protection system may be carried out since the beginning, on newly constructed structures (in such case, reference is often made to a “cathodic prevention system”) or as a retrofitting of older structures.
The anodes commonly used for the cathodic protection of reinforced concrete consist of a titanium substrate coated with transition metal oxides or other types of catalysts for anodic oxygen evolution. As the substrate it is possible to make use of other valve metals, either pure or alloyed. Pure titanium is, however, the largely preferred choice for the sake of cost. From the system design standpoint, the cathodic protection of a reinforcing frame may be carried out according to two distinct ways, that is with distributed or with discrete anodes. The protecting structure with distributed anodes provides covering the concrete cover surface of the reinforcement to be protected, suitably prepared, with anodes consisting of highly expanded meshes; the anodes are then covered with a few centimeter thick fresh cement layer. Alternatively, mesh or solid ribbons can be installed in conduits cut within the cover (whose depth is not sufficient to reach the iron), then filling said conduits with cement mortar. In newly constructed structures the anodes, typically anode mesh ribbons, can be installed directly over the reinforcing cage, kept electrically insulated from the iron by means of plastic or concrete-like spacers.
The anodic system is embedded in the structure at the time of casting the concrete for the construction. A slight direct current (typically from 1 to 30 mA per m2 of reinforcement) applied to the anodes, distributed along the whole structure, imposes a uniform cathodic potential to the reinforcement to be protected in case the latter has a sufficiently simple and regular shape. Conversely, if the reinforcement has a complex shape and presents some portions which are less accessible than others, or which have a different steel density per unit surface or other kinds of irregularities, it may be troublesome to ensure a sufficient protection to all of the reinforcement portions without providing an excess of current to other portions.
The discrete anode-type protection structure permits to overcome this inconvenience by using separate anodes, for instance in form of bars, plates, rods or segments of mesh or ribbon, installed in suitable holes or slits obtained in the concrete and cemented therein with cement mortar after their placement. The discrete anodes may be placed according to the needs, increasing their number or decreasing their spacing in those spots where it is necessary to provide more current. For some structures, a combination of mesh and ribbon anodes and of discrete anodes can be provided in order to obtain the best protecting effect. The maximum current density applicable to the above described type of anodes (mesh, ribbons or discrete anodes) is limited by the need of preventing an excessive concrete acidification in the surrounding zone. The latter in fact causes damages of several kinds, among which the build up of reaction products in a limited region around the anode and the consequent mechanical action which deteriorates the surrounding mortar with an inevitable steep rise in the interface resistance. The regulations in force provide a maximum current density per anode effective active surface (that is surface referred to the sum of the two faces) of 110 mA/m2 at most. Hence, to be able to supply the required protection current in compliance with the maximum current density, it is advisable to maximise the anodic surface per unit length without increasing too much the cost of materials as well as the installation cost associated with the manufacturing of deep holes or cuts in the concrete. Under another aspect, in order to restrict the installation costs it is also necessary to improve the ease of transportation and assembly of the anodes as much as possible. Finally, it is necessary to identify electrode geometries capable of increasing as much as possible the adhesion of the anode to the cement mortar used for their fixing. The electrode geometries of prior art discrete anodes evidence important deficiencies under all of these aspects, for instance because the anode surface increase per unit length may only be achieved by an increase in the diameter or length thereof. Moreover, the installation of cylindrical or of mesh or solid ribbon-shaped anodes may prove very difficult in vertical surfaces or on structure ceilings, where such anodes must be suitably anchored to the holes or slits obtained in the concrete before being covered with fresh mortar, to prevent them from falling under the action of gravity.